If ever there was a sworn devotee — a chanting, face-painted devotee — of all things big, it is Marvel.

So the prospect that Ant-Man, the minuscule Mighty Mouse of Marvel’s stable of powerhouse superheroes, might join the brawny big-screen ranks of the Hulk, Thor and the rest has long held some pleasing irony. That’s why lots of curiosity ensued when, just weeks before shooting on “Ant-Man” was to commence, Edgar Wright, the British blender of genre and comedy who had worked on the project for eight years, departed over “creative differences.” It was a sacrifice, seemingly, to the Marvel colossus.

The precise source of the dispute is unknown, but it’s clear enough from the final product, pushed forward with the quick insertion of director Peyton Reed (“Bring It On,” “The Break-Up”) and a rewrite by Adam McKay and others, that “Ant-Man” was bedeviled on one hand by staying true to its more modest size and idiosyncratic nature and on the other by meeting the larger, blander demands of being a Marvel movie complete with superhero cameos and sequel potential.

The result is a film that’s not quite sure of itself. It sometimes seems to be wearing clothes a size too big.

Paul Rudd plays Scott Lang, a politically motivated cat burglar who is being released after three years in San Quentin. He has an ethnically diverse group of petty criminal friends: Tip (T.I.) Harris, David Dastmalchian and Michael Pena, the only actor in the movie who’s rightly convinced he’s in a comedy. Lang is trying to right himself for the sake of his young daughter, Cassie (Abby Ryder Forston), and for the purpose of paying child support to his ex-wife (Judy Greer, an actress too good to be twice relegated to the domestic sidelines in this summer’s blockbusters).

But spryness (an essential quality for any movie about an insect superhero) is missing from these scenes. The movie is too controlled for Rudd’s goofball charm to break free.

Through some strained plot mechanics, Lang is recruited by the original Ant-Man, the scientist Hank Pym (Michael Douglas), to succeed him in the suit. Along with his daughter (a bob-sporting Evangeline Lilly), he’s conspiring to prevent a former apprentice (Corey Stoll) from unlocking the atomic secrets that led to Ant-Man’s secret power: the ability to shrink down to bug size yet maintain strength.

With the press of a button, he can toggle between big and small and make himself all but invisible. The perspective change makes for some unlikely superhero foes (such as a bathtub drain). During training, while Lang tries to perfect his communication with other underground ants, he sometimes pops out of the ground like a sprouted cabbage.

With a screenplay credited to Wright, Joe Cornish, McKay and Rudd, “Ant-Man” unfolds in pleasingly human-sized fashion. It’s a heist movie, one in which no city is leveled, and it seems to suggest that Marvel has gone on a diet.

It’s only in the climactic scenes that the movie unlocks the potential of its shape-shifting. Rather than taking place above the skyline of a metropolis, the big action scenes are set inside a briefcase and in Cassie’s bedroom. Such moments, sprinkled throughout, are like glimpses of a better “Ant-Man” that might have existed.

Change, we are told, is afoot in the Marvel Cinematic Universe. “Ant-Man” is the final movie in the studio’s Phase Two, and there are promises of bigger intergalactic battles looming in Phase Three.” Still, you have to squint pretty hard to spot the differences from Marvel movie to Marvel movie. If “Ant-Man” proves anything, it’s that any diversion in this universe is likely to get stomped underfoot.